1. Field of the Invention
The present invention relates to circuits and methods for read-enabling memory devices synchronously with the reaching of minimum functionality conditions of memory cells and reading circuits, particularly for non-volatile memories.
2. Discussion of the Related Art
In non-volatile memories with so-called static reading, it is possible to assuredly read a correct data item in the location addressed by the read address, even if initially the reading circuits have not reached their minimum functionality conditions.
Even if reading is started at a voltage level that is not yet sufficient to ensure assuredly correct reading of the data item, its correctness is in fact achieved nonetheless after a certain period of time, when the supply voltages assumes the steady state, i.e., when the supply voltage reaches at least the level of a threshold voltage of a generic non-volatile memory cell (approximately 2.1 V).
However, static-type reading approaches show difficulties in management for solving problems of noise generated by switching of outputs. Furthermore, these approaches do not easily allow to reduce consumptions.
In order to obviate this problem, solutions known as sampled and with dynamic (time-controlled) data sensing structures have been proposed.
The sampled-type reading approach entails reading the memory cell after a certain preset time and in this case it is indispensable that the read data item be actually correct, since dynamic reading does not allow modification of the data item at a later time.
Therefore, if reading is activated before the minimum functionality condition of a generic memory cell (voltage at least equal to the threshold voltage of a non-volatile cell) has been reached, the data item read in incorrect conditions remains incorrect until the next reading operation.
This drawback is particularly felt in memory devices that are meant to start up a more complex system, such as for example a computer, when the system enables the memory from the very start.
In this case, the first reading operation after device power-on and while the supply voltage level has not yet reached the steady state can have problems relating to the reliability of the read data. This occurs in all cases involving, for any reason, a transient situation of the power supply.